(1) Field of the Invention
The present invention relates generally to an air conditioning system mounted in an automotive vehicle compartment, and more specifically to an air conditioning system particularly mounted in an automotive vehicle which performs fixing control over the swing position of an air mix door irrespective of control operation by an automatic temperature control means, particularly when a maximum compartment temperature of the vehicle is required, for example, in a case when urgent defrosting is needed.
(2) Description of the Prior Art
An air conditioning system for a vehicle compartment has been proposed which performs automatic temperature control over the compartment temperature. In a warm air blow-out mode such as a defrosting operation, the air conditioning system fixes the air mix door at a full position for the air to flow through a heating means such as a heater core to maintain the temperature of the warm air blown out into the compartment through a ventilator, i.e., a defroster nozzle facing toward a windshield of the vehicle at the highest possible temperature so that the maximum defrosting effect can be achieved. In a conventional air conditioning system, there is provided a temperature-control switch for generating an electrical signal at the front output stage of an electronic control circuit to fix the air conditioning system in the maximum heating or cooling operation mode. The temperature-control switch is located within the electronic control circuit in parallel with a voltage comparator (temperature detecting circuit) and actuates either of a pair of transistors in the last output stage of the electronic control circuit depending on an actuating signal of a different power line from that of the voltage comparator and furthermore actuates a double solenoid vacuum valve connected to both transistors so that the maximum controlled vacuum output is generated.
In the defrost mode, or in a mode requiring the maximum capability of the air conditioning system, the double solenoid vacuum valve forces a vacuum power servomechanism to displace and fix the air mix door up to a maximum stroke position.
As in the case of the above-described prior art, when a lever or push button is operated to select the defrosting mode, an electric circuit for connecting a fixed resistor to such a temperature detecting circuit and a circuit for switching the resistor in defrosting mode are required, so that the whole circuit may become complicated.
When an electronic circuit in the air conditioning system fails, an emergency circuit is required which maintains, irrespective of a temperature control circuit, the basic function of the air conditioning system such as cooling and heating capacity.
In making up such an emergency circuit, conventionally a temperature-control switch is incorporated to perform fixing control of maximum cooling or maximum heating irrespective of an automatic temperature adjusting electronic circuit. In this case, an additional electric circuit and switch are required, so that the whole circuit may become complicated.
Such a conventional air conditioning system has a disadvantage that when the electronic circuit fails, the circuit as described above cannot serve as the emergency circuit for a failure of the subsequent stage of the automatic temperature adjusting electronic circuit.
Among other things, a conventional air conditioning system is designed to perform two operations, of defrost mode selection through a mode selector and of fixing a temperature setting lever into a maximum temperature range for performing a defrost operation in a short period of time, independently of each other, so that an operator must carry out an urgent defrost operation through a twice complicated operations.